STM32U575: Accessing System Bootloader Without PH3/BOOT0

[jtrach]

I have an STM32U575 MCU with the PH3/BOOT0 pin permanently pulled down to ground. I need to develop a process to flash a proprietary bootloader to fresh boards (ie. the flash is blank) with this configuration. I am currently using STM32CubeProgrammer to connect to a NUCLEO-U575ZI-Q with an external STLINK-V3 over UART/I2C to simulate this process. Even when the board has been reset and the flash is empty, the board does not jump to the system bootloader unless I drive PH3/BOOT0 high. Is the STM32U575 supposed to automatically run the system bootloader when the flash is empty, or will my inability to drive PH3/BOOT0 high on my product prevent me from accessing the system bootloader?

[Saket_Om]

Hello ​@jtrach 

On STM32 devices, the system bootloader in ROM is typically entered by a specific boot selection mechanism, classically involving:

• BOOT0 pin state
• boot configuration / option bytes

For the STM32U5 family, if BOOT0 is low, the MCU does not automatically decide:

“flash looks empty, so I’ll fall back to the ROM bootloader.”

Instead, it attempts the configured normal boot path.

If the flash is blank, the CPU may simply fail to execute valid application code rather than redirecting to the system memory bootloader.

[jtrach]

If we only have access to an I2C bus for initial factory programming (without SWD or J-Link), is pulling the BOOT0 pin high the only way to activate the internal bootloader and program the MCU?

What is STMicroelectronics' officially intended hardware and software workflow for this scenario?